1. Field of the Invention
The present invention relates to an AC-powered LED light engine able to gradually gear up and down the number and current of excited LED sub-arrays in accordance with the voltage level of the rectified sinusoidal input voltage, and able to smoothly dim up and down the extrinsic LED sub-arrays via a shared current sense and modulation unit while keeping the quasi-sinusoidal line current waveform in good shape as well as maintaining almost the same high Power Factor (PF) and almost the same low Total Harmonic Distortion (THD) throughout the entire dimming range.
2. Description of the Prior Art
LED-based lighting devices are gradually becoming the preferred lighting equipment because of having a longer lifetime to reduce maintenance cost, and being less likely to get damaged than legacy lighting devices.
Technically, an AC sinusoidal input voltage would normally be rectified into a rectified sinusoidal one before coming into use for the DC-driven LEDs. In the vicinity of the beginning and end of each DC pulse cycle (aka “dead time”) where the input voltage is less than the combined forward voltage drop of the LEDs, the LEDs cannot be forward-biased to light up. The dead time in union with the conduction angle constitutes a full period of the rectified sinusoidal input voltage. A longer dead time translates to a smaller conduction angle, and hence a lower power factor because the line current is getting too thin to be similar in shape to the line voltage.
Traditional LED drivers would usually come along with the following application problems. The first problem would be the need for a more complicated and more expensive driving circuit consisting of an EMI filter, a bridge rectifier, a short-life Power Factor Corrector (PFC), etc. to drive LEDs. The second problem would be the flicker phenomenon due to no current flow through the LEDs during the dead time. The third problem would be a lower power factor exhibited by a low-power PFC with a loop current too weak to be precisely sensed to correctly shape the AC input current into a sinusoidal waveform. If the loop current appears too low to be precisely sensed by the current-sensing circuitry in the PFC stage, the PFC would fail to properly keep the line current in phase and in shape with the line voltage to achieve a high PF. Often mentioned in the same breath with the issue of a low PF is the issue of a high THD. The THD resulting from the discontinuous or jumping points in the AC input current waveform would have much to do with the existence of the dead time.
Besides, traditional phase-cut dimming, be it leading-edge or trailing-edge TRIAC dimming, would achieve dimming function by means of cutting off some conducting phase from the line current waveform, leading to significant deterioration of PF and THD.